The present invention relates to a method of determining time-dependent boundary interface changes in a natural resource deposit and, more particularly, to a method by which measurements of the gravity gradient in the area of a natural resource deposit, such as a petroloeum reservoir, can provide information as to reservoir density changes.
Oil and natural gas hydrocarbon reservoirs form as a consequence of the transformation of organic matter into various types of hydrocarbon materials, including coals, tars, oils, and natural gas. It is believed that oil and gas reservoirs form as lighter hydrocarbon molecules percolate toward the surface of the earth until they are trapped in a relatively permeable layer beneath a relatively impermeable layer that ‘caps’ the permeable layer. The lighter hydrocarbon molecules continue accumulating, often accompanied by water molecules, into relatively large sub-surface reservoirs. Since the reservoirs exist at various depths within the earth, they are often under substantial geostatic pressure.
Hydrocarbon resources have been extracted from surface and sub-surface deposits by the mining of solid resources (coal and tars) and by pumping or otherwise removing natural gas and liquid oil from naturally occurring sub-surface deposits.
In the last century, natural gas and oil have been extracted by drilling a borehole into the sub-surface reservoirs. In general, most reservoirs were naturally pressurized by the presence of free natural gas that accumulated above the liquid oil layer and, often, by water that accumulated below the liquid oil layer. Since naturally occurring crude oil has a density lower than that of water (i.e., ranging from 0.7 in the case of ‘light’ crude oil to 0.9 in the case of ‘heavy’ crude oil), crude oil accumulates above the water-permeated layer and below the gas-permeated layer. Thus, a borehole terminating within the oil-permeated layer would yield oil that receives its driveout energy from an overlying gas-permeated layer and/or an underlying water-permeated layer.
In general, the ‘primary’ recovery of crude oil occurs during that period of time that the natural pressurization of a reservoir causes the crude oil to be driven upwardly through the well bore. At some point in the operating life of the reservoir, the naturally occurring pressurization is effectively depleted. Several different methods, known generally as secondary recovery methods, have been developed to extract crude oil after natural pressurization is exhausted. In general, secondary recovery involves re-pressurizing the reservoir with a fluid (i.e., a liquid or a gas) to lower the oil viscosity and/or drive the remaining crude oil in the oil-permeated layer to the surface through one or more wells. The drive fluid is introduced into the reservoir by injection wells which pump the pressurized drive fluid into the reservoir to displace and thereby drive the oil toward and to the producing wells.
Various schemes have been developed for the placement of the injections wells. For example, a line of injection wells can be placed at or adjacent to a known boundary of the reservoir to drive crude oil toward and to the producing wells. As the boundary between the pressurizing fluid advances past the producing wells, those producing wells can be capped or, if desired, converted to injection wells. In another arrangement, injection wells are interspersed between production wells to drive the oil in the oil-permeated layer away from the injection point toward and to immediately adjacent producing wells.
Various fluids, including water at various temperatures, steam, carbon dioxide, and nitrogen, have been used to effect the re-pressurization of the reservoir and the displacement of the desired crude oil from its rock or sand matrix toward the production wells.
In the waterflood technique, water at ambient temperature is injected into a reservoir to drive the oil toward and to the producing wells. The injected water accumulates beneath the crude oil and, in effect, floats the lighter density crude oil upwardly toward and to the borehole of the producing well. In those cases where the oil-permeated layer is relatively thin from a geological perspective and is also confined between two relatively less permeable layers (i.e., an impermeable reservoir ceiling and a more permeable reservoir basement), water is injected at a relatively high pressure and volume to effect an ‘edge drive’ by which the crude oil is pushed toward the oil producing wells. Sometimes, the injected water is heated to assist in lowering the viscosity of the oil and thereby assist in displacing the crude oil from the pores of the permeable sand or rock. The waterflood technique is also well-suited for driving natural gas entrapped within the pores of relatively low-permeability rock to a producing well.
In the steamflood technique, steam is used to displace or drive oil from the oil bearing sand or rock toward and to the producing wells. The steam, which may initially be superheated, is injected into the oil-permeated layer to cause a re-pressurization of the reservoir. As the steam moves away from its initial injection point, its temperature drops and the quality of the steam decreases with the steam eventually condensing into a hot water layer. Additionally, some of the lighter hydrocarbons may be distilled out of the crude oil as it undergoes displacement at the interface between the steam/hot water and the crude oil. The steam injection can be continuous or on an intermittent start-and-stop basis.
In addition to the use of water and steam to effect reservoir re-pressurization and the driveout of the crude oil toward the production wells, carbon dioxide and nitrogen have also been used for the same purpose.
One problem associated with water, steam, or gas driveout techniques is the identification of the boundary or interface between the driveout fluid and the crude oil. In an optimum situation, the boundary between the driveout fluid and the to-be-displaced crude oil would move in a predictable manner through the reservoir from the injection points to the production wells to maximize the production of crude oil. The geology of a reservoir is generally complex and non-homogeneous and often contains regions or zones of relatively higher permeability sand or rock; these higher permeability zones can function as low-impedance pathways for the pressurized driveout fluid. The pressurized driveout fluid sometimes forms low-impedance channels, known as ‘theft’ zones, through which the pressurized fluid “punches through” to a producing well to thereby greatly decrease the recovery efficiency.
The ability to identify the position of and the often indistinct interface or boundary between the to-be-displaced crude oil and the pressurized driveout fluid, to track the velocity and morphology of that boundary, and to effect control thereof would substantially enhance secondary oil recovery.
Various techniques have been developed for gaining an understanding of the configuration of the sub-surface geology of an oil-containing reservoir. The dominant technique involves seismic echoing in which a pressure wave is directed downwardly into the sub-surface strata. The initial interrogation wave energy is typically created by the detonation of explosives or by specialized earth-impacting machines. The interrogation wave radiates from its source point with its transmission velocity affected by the elastic modulus and density of the material through which it passes. As with all wave energy, the interrogation wave is subject to reflection, refraction, scattering, absorption, and dampening effects caused by the material through which it passes and from which it is reflected. The reflected wave energy is detected by geophones spaced from the seismic source point and subjected to processing to yield a model of the reservoir. This technique is highly developed and well-suited for detecting sub-surface structures that may be favorable to the accumulation of oil or gas.
Other techniques for investigating sub-surface geology include the use of gravimeters to detect minute changes in the magnitude of the gravity vector for the purpose of detecting sub-surface structures that may be favorable to the accumulation of oil or gas.
U.S. Pat. No. 6,152,226 issued Nov. 28, 2000 to Talwani, Feldman, and Schweitzer and in common assignment herewith discloses the use of gravity gradiometers to obtain information as to the gravity gradient at various data-taking positions on the surface above an oil reservior and the use of a subsequent set or sets of data to detect the location and morphology of the boundary or interface between the pressurized driveout fluid and the oil or natural gas in those reservoirs undergoing secondary recovery. The data can be subject to inversion processing in an effort to determine changes in the sub-surface density; however, inversion techniques can result in a sub-optimum local minimum rather than the desired global minimum. The efficient processing of the data can provide information for a manager to more efficiently direct the application of any driveout fluids to optimize recovery from both a time and economic standpoint.